chan_iax2: Reduce indentation in __attempt_transmit().

* Reduce indentation in __attempt_transmit().

* Don't update the static last error time variable every time in
__schedule_action() and socket_read().
........

Merged revisions 398456 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@398457 65c4cc65-6c06-0410-ace0-fbb531ad65f3